MDEV-6150 Speed up connection speed by moving creation of THD to new thread

Creating a CONNECT object on client connect and pass this to the working thread which creates the THD.
Split LOCK_thread_count to different mutexes
Added LOCK_thread_start to syncronize threads
Moved most usage of LOCK_thread_count to dedicated functions
Use next_thread_id() instead of thread_id++

Other things:
- Thread id now starts from 1 instead of 2
- Added cast for thread_id as thread id is now of type my_thread_id
- Made THD->host const (To ensure it's not changed)
- Removed some DBUG_PRINT() about entering/exiting mutex as these was already logged by mutex code
- Fixed that aborted_connects and connection_errors_internal are counted in all cases
- Don't take locks for current_linfo when we set it (not needed as it was 0 before)

mysys/thr_mutex.c

@@ -510,7 +510,7 @@ int safe_cond_wait(pthread_cond_t *cond, safe_mutex_t *mp, const char *file,
     fprintf(stderr,
 	    "safe_mutex:  Count was %d in thread 0x%lx when locking mutex %s "
             "at %s, line %d\n",
-	    mp->count-1, my_thread_dbug_id(), mp->name, file, line);
+	    mp->count-1, (ulong) my_thread_dbug_id(), mp->name, file, line);
     fflush(stderr);
     abort();
   }
@@ -564,7 +564,7 @@ int safe_cond_timedwait(pthread_cond_t *cond, safe_mutex_t *mp,
     fprintf(stderr,
 	    "safe_mutex:  Count was %d in thread 0x%lx when locking mutex "
             "%s at %s, line %d (error: %d (%d))\n",
-	    mp->count-1, my_thread_dbug_id(), mp->name, file, line,
+	    mp->count-1, (ulong) my_thread_dbug_id(), mp->name, file, line,
             error, error);
     fflush(stderr);
     abort();